Common types of such machines operating as compressors contain a pair of rotors of the kind mentioned above running in a working space formed by a surrounding casing. The casing has a shape of two intersecting cylinders, each cylinder housing one of the rotors. Through the helical lobes and grooves, the rotors intermesh, forming chevron-shaped working chambers between themselves and the surrounding casing, in which working chambers a gaseous fluid is transported and compressed. One of the rotors is a male rotor, having the major part of each lobe located outside its pitch circle and the other rotor is a female rotor, having the major part of each lobe located inside its pitch circle.
A screw rotor of this kind is a complicated element, in particular concerning its geometry. In a transverse section through a rotor, each flank of each lobe follows a curve which usually is of complex structure and has to cooperate with a corresponding complex curve on a lobe of the other rotor. In many cases, the lobes are unsymmetrical so that the two flanks of a lobe are different. Since the lobes are helically extending along the rotor, the angular position of a transverse section through the rotor varies if the location of the section is moved axially along the rotor. The rotors intermesh along their entire axial extension, whereby the sealing lines forming the working chambers each receives a complicated three-dimensional configuration. For attaining an optimal shape of the rotor profiles, considerations have to be made concerning clearances, leakage, efficiency, capacity, noise, vibrations and manufacturing aspects. Due to the complex geometrical structure of the rotors, the different requirements they must meet and the high accuracy that is necessary, the costs for their manufacture are considerable.
According to the conventional technique, a rotor is machined from cylindrical steel blanks in a plurality of passes. This process is time-consuming and results in waste of material.
In the field of screw rotor technology, there have been continuous efforts to attain rotors which are more economical to manufacture. This has resulted in many solutions using different manufacturing techniques and different materials including composite rotors. Examples of rotors made by less conventional manufacturing techniques and/or other materials and material combinations can be found in the following patent documents:
U.S. Pat. No. 3,918,838, which discloses a rotor in which a plastic is molded on a metal skeleton,
West German DE 2 409 554, which discloses a rotor made by glass fiber-reinforced plastic on a steel shaft,
West German DE 3 424 148, which discloses an injection-molded polymer rotor,
West German 2 364 727, which discloses a rotor built by a plurality of plates,
International Publication No. WO 86/05555, which discloses a plastic rotor,
U.S. Pat. No. 2,714,314, which discloses a rotor made as a welded construction,
British Patent Specification No. 1,433,132, which discloses manufacturing of a rotor by extruding, and
West German DE 3 124 247, which discloses a ceramic rotor made of silicon nitride.
The objects of the present invention are to achieve a rotor which can be more economically manufactured than by the conventional technique, and to develop a process for manufacturing such a rotor.